As a method for forming a color photograph or a color print, there is a method for forming an image, such as a picture or a character, on a photosensitive sheet by exposing the sheet. There are different types of photosensitive sheets, for example, a photosensitive sheet employing a multi-layer color development method, in which sheet three or more layers of photosensitive emulsions with different color sensitivities are layered on a single supportive sheet thus forming a photosensitive member, a photosensitive sheet that employs a film in which each emulsion layer contains a pigment and a developing agent so that the film is capable of being exposed and developed simultaneously, and the like. A still another photosensitive sheet is a so-called Cycolor medium, which employs, as a photosensitive material, microcapsules (cyliths) that contain a chromogenic substance and a photoinitiator. In the Cycolor medium, a thin supportive body formed from, for example, polyester, is coated with numerous cyliths of a very small size. When exposed to light, cyliths harden so that only the cyliths of a specific color are activated, and the cyliths are ruptured by pressurization, thereby forming a predetermined image of a specific color. Other photosensitive sheets have different color development principles, but are exposed to light of the color of an image or its complementary color to form an image.
In widely used methods for exposing a photosensitive sheet as described above, white light is split into three primary colors by a filter or the like, and images are formed using the individual primary colors, and then combined to form an image of predetermined colors or an image of their complementary colors on the photosensitive media. Another technology has recently been developed, as disclosed in Japanese patent application laid-open Nos. Hei 5-211666 and Hei 5-278260, in which LEDs or lasers that emit red, green and blue light are employed as light-emitting sources, and the light-emitting sources are controlled so that an image of predetermined colors is formed on a photosensitive sheet and the sheet is thereby exposed.
Employment of LEDs or the like as light sources (light-emitting elements) for individual colors allows a compact construction of the light sources. Furthermore, provision of the light sources for individual colors enables control of exposure duration, brightness and the like. For a photosensitive sheet employing photosensitive materials having different exposure characteristics for individual colors, a design has been considered in which the exposure duration and brightness can be set appropriately for each color, whereby it will become possible to form images with good color balance and reduced color distortion on a medium in which the photosensitive materials have different exposure characteristics to different colors.
A printing apparatus 10, as shown in FIG. 1, equipped with an exposure apparatus having LEDs or lasers as light-emitting elements, has been developed. In the printing apparatus 10, an exposure head 15 is mounted on a carriage 13, and the carriage 13 is moved along a shaft 12 in scanning directions X. A photosensitive sheet 1, such as a Cycolor medium, is conveyed by a sheet-conveying roller 11 in a predetermined direction (sheet-conveying direction) Y. The photosensitive sheet 1 is thereby moved relative to the light-emitting elements of the exposure head 15, so that the entire photosensitive sheet is exposed to light from the LEDs or lasers of the exposure head 15 to form an image.
FIGS. 2 shows an example of the arrangement of the exposure head 15, especially relating to the exposure head 15 having LEDs 31 for red color (R). In the exposure head 15, exposure is performed by light-emitting sources 21 each having four LEDs 31 for a single color. Each LED 31 of the light-emitting source 21 emits light when power is supplied thereto from a power supplying circuit 23. The light emitting duration (exposure duration) of each LED 31 is controllable by a switching circuit 22. The power supplying circuit 23 has a constant voltage power source 34, and semi-fixed resistors 33 for setting the power supplied to the corresponding LEDs 31 in accordance with the brightness of the corresponding LEDs 31. During assembly in a factory or immediately before shipment, the semi-fixed resistors 33 are adjusted so that the brightness of the red (R), green (G) and blue (B) light emitting sources become predetermined values at the time of shipment. Normally, the individual variation of LEDs is large and the variation in amount of light emission (brightness) is large, and many photosensitive sheets have different sensitivities depending on wavelengths. Therefore, the semi-fixed resistors 33 are provided in the power supplying circuit 23 to set power supplied to each LED in accordance with the variations of LEDs and basic characteristics of photosensitive sheets depending on wavelengths.
In the switching circuit 22, a transistor switch 32 is provided for each LED 31. A CPU 25 controls a timing generating circuit 24 on the basis of pixel gradation information. Each transistor switch 32 is turned on and off by a signal from the timing generating circuit 24. Therefore, the duration of power supply to each LED 31 is controlled by the signal from the timing generating circuit 24, so that the duration of exposure of a photosensitive sheet can be controlled separately for each color and each LED 31.
As described above, while it has a simple circuit arrangement, the exposure head 15 shown in FIG. 2 is able to absorb or offset the individual variations of the LEDs by using the semi-fixed resistors. Therefore, it is possible to provide initial settings in accordance with the basic characteristics of a photosensitive sheet to be exposed. However, since the forward voltage of the LEDs 31 and the voltage between the collector and emitter of each transistor switch 32 changes depending on temperature, the value of drive current supplied to the LEDs 31 changes with changes in ambient temperature or operating conditions of the exposure head 15. As a result, the color tone of an image formed by exposure may vary or uneven color development may occur, depending on the operating conditions.
FIG. 3 shows another example of the exposure head 15 having LEDs. In this exposure head 15, constant current circuits 35 are provided in a power supplying circuit 23 corresponding to LEDs 31 provided in a light emitting source 21, so that the initial setting of the current to be supplied to the LEDs 31 from the constant current circuits 35 can be made using the semi-fixed resistors 33. Therefore, a constant current is supplied to each LED 31 despite changes in temperature depending on operating conditions, so that images with relatively stable color tones can be obtained.
However, in the exposure head 15, the semi-fixed resistors, provided in the same number as that of the LEDs, are adjusted at a timing when the initial setting of the semi-fixed resistors is possible, such as at the time of shipment. Once adjusted, the semi-fixed resistors cannot be re-adjusted. Therefore, for changes of the environmental conditions (printing environments) which occur despite maintenance of constant current values, for example, fluctuation of brightness of LEDs depending on temperature, sensitivity variations of a photosensitive sheet, temperature-dependent sensitivity changes of the photosensitive sheet, lightness changes or color tone changes made in accordance with user's taste, and the like, this exposure head 15 also requires that the duration of exposure by the LEDs be changed by controlling the switching circuit 22 during operation of the printing apparatus to achieve corrections for changes in brightness of LEDs and the like.
The duration of exposure by LEDs is controlled by the switching circuit 22, the CPU 25 and the timing generating circuit 24, and can be used as a basis for corrections for changes in printing environments and gradation control if the gradation level is as low as about several tens of gradation increments. However, since graduation control at a level of about 256-1024 gradation increments is recently required, a control mechanism with a resolution higher than the graduation level is needed in order to perform gradation control and corrections for changes in brightness and the like on the basis of exposure duration. Therefore, the control mechanism, the exposure head and the printing apparatus become very expensive. As the gradation level increases, such cost increases become greater. To solve this problem, it is possible to use LEDs that have reduced individual variations and stable temperature characteristics and therefore requires brightness adjustment to a less extent, or to use photosensitive sheets in which the characteristic variation among lots is small. This may curb the cost increase of the exposure duration control mechanism, but results in cost increases of LEDs and photosensitive sheets.
If corrections for environmental changes are performed on the basis of exposure duration, the exposure duration per exposure cycle of a single dot becomes short when, for example, the LED brightness is increased. Therefore, if the gradation level becomes high, it becomes impossible to secure a sufficient resolution in the direction of the time axis for graduation control, so that sufficient graduation expression cannot be obtained. Then, various problems arise. That is, if the correction is simplified in order to obtain sufficient gradation expression, the image resolution decreases depending on temperature or the sensitivity of photosensitive sheets. Conversely, if the gradation level control is simplified, desired color expression may not be obtained. It is possible to increase the exposure duration in order to secure a certain resolution. However, an increase in exposure duration per dot will considerably increase the time required for printing.
Accordingly, it is an object of the invention to provide an exposure apparatus, an exposure method and a printing apparatus that always allow easy correction for changes in environmental conditions that include temperature regarding light-emitting elements, such as LEDs, sensitivity of photosensitive sheets, and, in addition to correction for changes in environmental conditions, provide sufficient gradation expression. It is an another object of the invention to make it possible to provide an exposure apparatus and a printing apparatus that have a function for correction regarding environmental conditions and a function capable of gradation expression at a high gradation level. It is still an another object of the invention to provide an exposure apparatus and a printing apparatus that perform printing with high image quality without requiring expensive LEDs or a photosensitive sheet which have stable characteristics.
It is a further object of the invention to provide an exposure apparatus and an exposure method that allow easy initial setting of light-emitting elements using a simple circuit arrangement and that allow a user to easily correct or adjust the conditions of exposure by light-emitting elements even after shipment from a factory.